Why is the 'GPU compute' render option blurred out for me?
Press ctrl + alt + u to open the "User Preferences".

Switch to the "System" tab and change your compute device.



(I do not have a compatible GPU, but yours should show up here, if it is compatible.)

I recommend clicking "Save User Settings" so that this is saved for next time you open a new Blender file.

Your GPU should now appear under the GPU compute section.